Harry Reid doesn’t rule his Senate Democrats with an iron fist. For the most part, the majority leader is fine with his colleagues speaking their minds and being flexible in their votes. He asks for party discipline only when it matters. And with the rollout of President Obama’s signature health law facing severe turbulence, it matters now.

Signs of unrest abound. Most prominently, Sen. Mary Landrieu, D-La., is backing a legislative fix to the health program’s canceled insurance policies, despite Obama’s insistence that he’s handling it administratively. Still, there has been no open rebellion. Reid’s troops are complaining about the president; they’re not setting out to embarrass their leader.

Even Republicans are impressed by how Reid’s troops are falling in line. “Frankly, I’m amazed that they stick together as well as they do, because it’s not in their best interests to follow some of the things that the leadership demands of them,” said Sen. Orrin Hatch, R-Utah.

During the recent government shutdown, Senate Democrats unanimously swatted away bill after bill authored by House Republicans, even as the measures were designed to embarrass the Democrats and feed into 2014 attack ads. Reid’s leadership looked more remarkable when compared with the unrest on the other side of the Capitol, where House Speaker John Boehner has muddled through a series of internal defections and insurrections in the last year.

There’s nothing particularly fanciful about Reid’s tactics. The Nevadan grinds out long hours listening to his party’s senators, often taking their calls or drop-ins in the midst of other meetings. He shields them from as many difficult votes as possible.

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Most important, he gives almost total latitude to senators who are next up on the ballot.

“The first people you give passes to are the most vulnerable,” said a senior Senate Democratic leadership aide. It’s no coincidence that the only four defections when Senate Democrats passed a budget earlier this year were members facing reelection in 2014 in states that Mitt Romney carried. Those who had just won in such states—Sens. Heidi Heitkamp of North Dakota, Claire McCaskill of Missouri, and Jon Tester of Montana—carried the voting load. “When you’re up, you’re up. When you’re not up, you’re to help those who are up,” the leadership aide said.

To hear Sen. Brian Schatz of Hawaii tell it, Reid spends a lot of his time as a social worker counseling needy senators, each with his or her own specific problem—a tough race back home, a contested primary (as in Schatz’s case), or a particular home-state industry that needs a boost.

“It’s member management and making sure that everybody’s home-state priorities and political predispositions are accounted for, and that’s no small task,” Schatz said. “He’s been extraordinary in his ability to keep us together.”

Reid’s No. 1 unwritten rule: no surprises. Sen. Max Baucus of Montana violated that edict back in March when he voted against his party’s budget—the measure passed 50-49—despite chairing the Senate Finance Committee. Reid’s team didn’t know Baucus’s no vote was coming, aides have said; they were even more taken aback when Baucus announced his retirement a month later: He had voted against the budget and wouldn’t even be facing the electorate again.

Chris Kofinis, a former chief of staff to Joe Manchin of West Virginia, perhaps the Senate’s most moderate Democrat, said Reid’s operation never once punished Manchin for veering from the party line. “There was always understanding about what you needed to do,” Kofinis said.

Kofinis said Reid and his top adviser, Chief of Staff David Krone, were always available to talk ahead of key votes. “They build support the old-fashioned way,” he said. “They earn it.”

Republicans say Reid has won his troops’ loyalty by contorting the Senate’s rules to stifle GOP amendments at will. Just this week, as the Senate debated the defense authorization bill, Republicans sought to have more of their amendments considered. Reid said no. “Everyone has to understand that this is not going to be an open amendment process,” Reid said. “It is not going to happen.”

However Reid built his party’s unity, the combination of the disastrous rollout of the health reform website—already 39 Democrats in the House have voted for a Republican measure aimed at nicking the law—and the looming fight over rewriting the Senate’s rules to curb future filibusters could prove Reid’s biggest test yet.

Democratic senators are lining up with legislative fixes for Obamacare. Manchin is supporting a GOP bill to delay the individual mandate for a year. Jeanne Shaheen of New Hampshire has pushed legislation to delay the enrollment deadline by two months or more. And Alaska’s Mark Begich wants to create a new, cheaper “copper” health care option. Then there’s Landrieu’s bill to let people keep their canceled health plans.

While red-state Democrats who talk about those bills—and score political points by proposing them—have Reid’s blessing, actually voting on them does not. None of the measures are currently scheduled for floor time. Reid is keeping them at bay until at least December.

How successful Reid can be beyond that could depend on whether the Obama administration cleans up the health care mess. If the website woes, insurance cancellations, and tales of doctors axed from insurance plans drag into the election year, Reid may be forced to accommodate his vulnerable incumbents. Whatever happens, lawmakers say Reid has the instincts to maintain his governing majority.